The polyamine/epoxide resin systems known in the prior art are characterised for example by excellent adhesion to metals, very good resistance to chemicals and outstanding anticorrosive properties. With solvent-containing formulations and powder coating systems crosslinked films of high flexibility can be obtained by the use of epoxide resins of high molecular weights and/or polyaminoamides, for example based on dimeric fatty acids as hardeners. Coatings based on solvent-free liquid resins and solvent-free amine hardeners are very brittle on account of the low molecular weights of the epoxide resins and the resulting high network density. Tar substitutes such as coumarone resins for example are therefore used for plasticisation purposes in solvent-free formulations. Especially when using relatively large amounts of hydrocarbon resins such coatings however tend to undergo long-term embrittlement as a result of migration of the non-functional constituents.
A good and permanent elasticisation of the epoxide resins can be achieved by combination with polyurethanes. Thus, high molecular weight, amine-terminated polyether urethane ureas have been produced by reaction of prepolymers containing free isocyanate groups with amines in highly dilute solutions and then hardened with epoxide resins, as described for example in DE-A 23 38 256. The use of the solvents, in particular aromatic solvents, necessary for this purpose is however disadvantageous in practice both for technical as well as physiological reasons. On the other hand the viscosity of the solvent-free reaction products, such as have been specifically produced according to DE-A 23 38 256, is too high for practical applications.
A process for the production of elasticised moulded parts and sheet materials is described in DE-A 24 18 041, in which epoxide compounds are reacted with amine compounds that are obtained by hydrolysis of prepolymeric ketimines or enamines. Chemically resistant and thermally adhering thermosetting materials having improved properties can be produced by this process. The disadvantage of the described process is the high process technology expenditure.
DE-A 21 52 606 describes reactive systems based on alkylphenyl-blocked polyisocyanates and polyamines, which optionally may also be hardened in combination with epoxide resins. These reactive systems too have some disadvantages as regards application technology, for example the reactive systems have a relatively high viscosity and the blocking agent that is released is of comparatively low molecular weight, which means that over time it migrates from the coating and the adhesion of the coating to the substrate is no longer sufficient.
In order to allow a targeted reaction of polyisocyanate prepolymers with excess amounts of diamines it has previously been proposed many times to use polyisocyanates in block form as described for example in CA-A 1 219 986, EP-A 293 110 or EP-A 082 983. There, phenols or substituted phenols are used as preferred blocking agents. After reaction with the polyamines, these substances however cannot be removed or can be removed only incompletely from the reaction mixture on account of their high melting points. The possibly substituted phenols remaining in the mixtures or in the plastics composition then lead however to the already described disadvantages.
In EP-A 0 457 089 on the other hand secondary amines preferably having low boiling points are used as blocking agent. If these amines remain in the reaction mixture after the deblocking, this readily produces an objectionable odour. Although the secondary amine after use in epoxide systems may in principle be incorporated into the system, this reaction however proceeds relatively slowly especially at low temperatures (e.g. room temperature), as a result of which part of the amines leaves the coating. In a particularly preferred use the amine blocking agent is distilled from the reaction mixture after the deblocking. Although this procedure does yield products without an objectionable odour, it is however extremely complicated and thus costly.
From U.S. Pat. No. 6,060,574 reactive compositions are furthermore known that consist of reversibly blocked organic polyisocyanates and at least one polyamine having at least two primary amino groups and that optionally in addition also contain compounds containing oxirane groups. Hydrocarbon resins with phenolic OH groups are used as blocking agent for the organic polyisocyanates. Such blocked polyisocyanates are characterised by a significantly reduced reactivity to polyamines, compared with alkylphenol-blocked polyisocyanates. Prepolymers that are obtained by reacting polyhydroxy compounds with an excess of diisocyanates or polyisocyanates may be used as organic polyisocyanates. Polyether polyols that are obtainable by alkoxylation of suitable starter molecules (for example monomeric polyols) may for example be used as polyhydroxy compounds. The described polyether polyols frequently have the disadvantage however that, due to the production process, the products are basic or acidic (start of the alkoxylation for example with KOH, followed by addition of acid). This can have a disadvantageous effect on the aforedescribed reactive systems and may for example significantly reduce the storage stability. Furthermore the polyether polyols started with bases generally contain a large amount of unsaturated terminal groups and have a broad molecular weight distribution. The unsaturated terminal groups that are formed by the base-catalysed isomerisation of propylene oxide to allyl alcohol cause chain termination and a reduction of the OH functionality. The reduced OH functionality due to the unsaturated terminal groups is a serious disadvantage both for the mechanical properties as well as the compatibility in the reactive systems.
The object of the present invention was accordingly to provide new blocked polyurethane prepolymers that can be processed without any problem to form reactive systems that are solvent-free and harden at room temperature. The coatings produced from these reactive systems should have a good adhesion, in particular wet adhesion, resistance to chemicals, resistance to impact and shock, combined at the same time with flexibility and elasticity. In this connection the blocked polyurethane prepolymers should be particularly highly compatible with the other constituents of the reactive system.